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ISTANBUL, Turkey — Bombs shattered a bus Thursday and exploded outside a hotel where President Bush is to stay this weekend, back-to-back attacks that killed four people, wounded 17 and heightened security concerns over an upcoming NATO summit.

Authorities said militant leftists were suspected in the blasts in Istanbul, which killed four people and wounded 14, and an earlier attack outside a luxury hotel in Ankara that injured three others, including two police officers. Three suspects were detained in Istanbul, police said.

The blasts were the latest in a series of explosions — most of them small, without casualties — ahead of the summit. Bush arrives in Ankara on Saturday night to meet with Turkish leaders before heading to the summit in Istanbul.

Security is expected to be extremely tight at the Istanbul summit, with more than 23,000 police on duty. Police are using steel and concrete barriers to seal off a zone in the heart of the city, where the conference will take place. Military surveillance aircraft are to monitor the airspace above the meeting.

Turkey described as ‘strong and secure’
“Turkey is a strong and secure country. Such incidents take place in New York, in Washington and everywhere else in the world,” Foreign Minister Abdullah Gul said after the Ankara blast.

The White House pledged the attacks would not affect Bush’s travel plans.

“It does appear that these terrorist attacks are intended to disrupt preparations for the upcoming NATO summit, which is a gathering of free nations united in our global fight against terrorism,” said White House press secretary Scott McClellan. “In terms of (Bush’s) schedule, nothing has changed.”

Guler said the blast was caused by a concussion grenade — a type of bomb that makes a lot of noise but usually causes little damage unless it goes off in a confined area.

Witness Onur Kaval was coming from a pharmacy when he heard the blast in Istanbul.

‘It was a horrible scene’
“I saw smoke coming out of the bus,” Kaval said. “I saw people jumping out of the windows. Everyone was crying and was in shock. It was a horrible scene. I saw people without arms and whose feet were ripped off.”